196 
BULLETIN OF THE BUREAU OF FISHERIES 
An inconsiderable bacterial population was always present in the cultures. The 
bacteria were examined at intervals by plating them in agar medium prepared 
according to Waksmann’s formula (1,000 cc sea water, 1 g peptone, 1 g glucose, 0.5 
K 2 HP0 4 , 2-3 drops of 10 percent FeCl 3 , and 15 g agar; sterilized in autoclave at 15 
lbs. for 20 minutes). 
When the diatoms and the surrounding medium are relatively free of bacteria 
the Nitzsc.hia cells stay in suspension. When they tend to fall to the bottom and form 
loosely aggregated masses, microscopical examination shows them to be weighted 
with adherent bacteria. Plating under such conditions invariably shows a high bac- 
terial count. All subcultures from the stocks were made, therefore, by carefully 
decanting only top portions containing diatoms in suspension. 
At the beginning of each experiment all the flasks were inoculated with equal 
volumes of stock culture and 1 or 2 of them measured to determine the initial diatom 
population. Thereafter the cultures were measured every other day. At least 3 
control and 3 experimental flasks were used in each test. In many experiments this 
number was doubled. All the figures given in tables or graphs are averages of three 
or more samples. 
EFFECT OF HEAVY SURFACE LAYER OF OIL ON NITZSCHIA CULTURE 
In this set of experiments Nitzsc.hia was grown in 500 cc Erlenmeyer flasks con- 
taming 250 cc of culture covered by 25 cc of oil. The oil was sterilized in a boiling 
water bath for 1 hour. 
There was no indication that the presence of oil kills the Nitzschia and no 
immediate effects on its growth were discernible even when the oil was thoroughly 
shaken up with the cultures several times a day. Microscopic examination showed 
no signs of oil sticking to the surface of the diatoms. For approximately a week the 
experimental cultures showed no significant difference in their growth as compared 
with the controls. Their further propagation was, however, markedly inhibited. 
Photoelectrical measurements of the cultures presented considerable difficulties as 
it was some times impossible to withdraw a portion of the sample free from oil globules. 
I 11 two experiments, however, this difficulty was overcome. The first experiment, 
lasting 18 days, began on March 13 and was discontinued April 4, 1934. The second 
one, started on April 7, continued for 25 days until May 4. During the last experi- 
ment tests were made with purified mineral oil (Russian oil) and cod-liver oil. Cul- 
tures covered with cod-liver oil perished on the fourth day, those with mineral oil 
continued to the end of the experiment. The retarding effect of oil was noticeable 
in both groups, although in the cultures under Russian oil it was somewhat less 
pronounced than in those kept under crude Pelto oil (Fig. 12.) The adverse effect 
of oil can be measured by determining the percent of retardation of growth in the 
experimental flasks as compared with their growth in the controls. In the first experi- 
ment (March 13, Pelto oil) the retardation at the eighteenth day amounted to 35 
percent. In the second experiment Casting 25 days) retardation due to presence 
of Russian oil and Pelto oil was 37 and 42 percent, respectively. 
An inhibitory effect of a heavy surface layer of oil on the diatom culture was quite 
visible in many other samples which could not be measured. This can be seen in 
the photographs (fig. 13) taken 30 days after the beginning of one of the experiments 
in February 1934. The control culture (left) had numerous diatoms and therefore 
